This invention relates to heat-shrinkable plastic films, and more particularly, to reactor grade copolyester compositions useful as materials to make heat-shrinkable plastic films.
Heat-shrinkable plastic films are used as coverings, to hold objects together, and as an outer wrapping for bottles, cans and other kinds of containers. For example, such films are used for covering the cap, neck, shoulder or bulge of bottles or the entire bottle; for the purpose of labeling, protection, parceling, or increasing the value of the product; and for other reasons. In addition, such films may be used as a covering to package such objects as boxes, bottles, boards, rods, or notebooks together in groups, and such films may also be attached closely as a wrapping. The uses mentioned above take advantage of the shrinkability and the internal shrink stress of the film.
Poly(vinyl chloride) (PVC) films dominate the shrink film market. However, polyester films have become a significant alternative because polyester films do not possess the environmental problems associated with PVC films. Polyester shrink films ideally would have properties very similar to PVC films so that the polyester can serve as a xe2x80x9cdrop-inxe2x80x9d replacement on existing shrink tunnel equipment. PVC film properties that are desired for duplication include the following: (1) a relatively low shrinkage onset temperature, (2) a total shrinkage which increases gradually and in a controlled manner with increasing temperature, (3) a low shrink force to prevent crushing of the underlying container, (4) a high total shrinkage (for example, 50% or greater) and (5) an inherent film toughness so as to prevent unnecessary tearing and splitting of the film prior to and after shrinkage.
In U.S. Pat. No. 5,859,116 (the ""116 patent), a heat-shrinkable polyester film is made from a copolyester blend of 1 to 98.5 weight percent 1,4-cyclohexanedimethanol modified poly(ethylene terephthalate) (PETG copolyester), 98.5 to 1 weight diethylene glycol modified poly(ethylene terephthalate) (DEG modified PET copolyester) having a b* value of less than 15, 0.5 to 3 weight percent anti-blocking agent, and optionally, 5 to 15 weight percent crystallizable polyester, such as poly(ethylene terephthalate) (PET). The PETG copolyester has a dicarboxylic acid component of at least 95 mole percent terephthalic acid (TA) and a diol component of 65 to 80 mole percent ethylene glycol (EG) and 35 to 20 mole percent 1,4-cyclohexanedimethanol (CHDM). The DEG modified PET copolyester has a dicarboxylic acid component of at least 75 mole percent terephthalic acid and a diol component of 10 to 50 mole percent diethylene glycol (DEG) and 50 to 90 mole percent ethylene glycol. By varying the amount of diethylene glycol content in these copolyester blends, heat shrinkable films made therefrom can be produced with desired shrinkage characteristics, such as low-onset temperature similar to polyvinyl chloride (PVC). The use of the DEG modified PET copolyester having improved clarity also results in shrink films that are free of haze with a b* color value of 0.9 to 1.10.
While the shrink films of the ""116 patent provide many benefits over the prior art, some significant disadvantages are associated with the use of these blends of copolyesters. The blend formulation may contain up to four components, which include PETG copolyester, DEG modified PET copolyester, PET and an anti-blocking agent. Multiple component blends make processes during production for drying, weighing, blending, and extruding more complicated and labor intensive with each additional component. For example, the PETG copolyester needs to be dried at 150xc2x0 F., the PET at 300xc2x0 F., and the DEG modified copolyester at 110xc2x0 F. Thus, three dryers are required to produce the blend. In addition, a precise blending of all materials is critical in controlling the quality of the film since variations in composition affect the properties. The blending of amorphous polymers, such as PETG copolyester and DEG modified PET copolyester, with a crystalline polymer, such as PET, may also create problems in extrusion because the materials have different melt properties. The mismatch in melting can cause mixing inconsistency in extrusion. In addition, blending, material separation, inventory, and storage of the many materials can be tedious.
Thus, there exists a need in the art for a polyester material capable of use as a shrink film material that eliminates excessive handling requirements, makes drying more efficient, and produces a more consistent product. Accordingly, it is to the provision of such material that the present invention is primarily directed.
A reactor grade copolyester composition unexpectedly has a higher ductility as compared to a blend formulation, as disclosed in U.S. Pat. No. 5,859,116 (the ""116 patent), of a PETG copolyester and a DEG modified PET copolyester, both having the same mole percentages of monomers. The reactor grade copolyester composition and a shrink film made therefrom comprise a diacid component comprising residues of at least about 90 mole percent terephthalic acid and a diol component that is one of two compositions. Diol component (a) comprises residues of about 72 to about 88 mole percent ethylene glycol, about 10 to about 15 mole percent 1,4-cyclohexane-dimethanol, and about 2 to about 13 mole percent diethylene glycol. Diol component (b) comprises residues of about 59 to about 77.5 mole percent ethylene glycol, about 15 to about 28 mole percent 1,4-cyclohexanedimethanol, and about 7.5 to about 13 mole percent diethylene glycol. The diacid component and the diol component are each based on 100 mole percent.
In another embodiment, a heat-shrinkable film prepared from a reactor grade copolyester composition has a shrink curve almost identical to that of PVC film. The diacid component of the reactor grade copolyester composition comprises residues of at least about 90 mole percent terephthalic acid. The diol component of the reactor grade copolyester composition comprises residues of about 61 to about 82 mole percent ethylene glycol, about 16 to about 26 mole percent 1,4-cyclohexane-dimethanol, and about 2 to about 13 mole percent diethylene glycol within +/xe2x88x922.5 mole percent of the formula:
% DEG(optimum)=63xe2x88x924.26*(% CHDM)+0.0822*CHDM2.
In still another embodiment, a reactor grade copolyester composition comprises a diacid component comprising residues of at least about 90 mole percent terephthalic acid and a diol component comprising residues of about 52 to about 88 mole percent ethylene glycol, about 10 to about 28 mole percent 1,4-cyclohexanedimethanol, and about 2 to about 20 mole percent diethylene glycol, based on 100 mole percent diacid component and 100 mole percent diol component. The copolyester composition has a b* color value of xe2x88x921.0 to less than 4.5, which is an unexpected, significant improvement over the b* color value of the blend formulations of the ""116 patent.